1. Industrial Field of the Invention
This invention relates to an optical disk, and more particularly to an optical disk excellent in durability.
2. Related Art
In optical disks exclusively used for sound and/or picture reproducing, polycarbonate (PC) or polymethyl methacrylate (PMMA) has hitherto been used as a transparent substrate material, with vapor-deposited aluminum film as a reflecting layer.
The prior optical disks, however, have a problem that the reflecting aluminum film is susceptible to oxidation and hence its reflectivity decreases in the lapse of several years, until the disk becomes apt to cause insufficiency in reproducing ability.
It is known that an optical disk having a vapor deposited film of gold, resistant to oxidation, instead of aluminum film as reflecting layer is excellent in durability. However, vapor deposition of gold is industrially disadvantageous because the raw material is expensive.
As reflecting films for particular wavelength regions, multi-layer vapor deposition films of metallic oxide, metallic fluoride, metallic sulfide and the like are known. They are laminated on glass or PMMA substrate and used as an optical mirror, wavelength-selecting filter, or the like. Apart from the above, a method for preventing warpage of disk by providing a similar multi-layer vapor deposition film on PC substrate is also known (Japanese Patent Application Kokai (Laid-Open) No. 62-60142). These multi-layer vapor deposition films are superior in reflectivity to vapor deposition films of aluminum or gold, and are stable to oxidation. However, these films are disadvantageous in that they are inferior in adhesive property to PMMA and PC and poor in durability.
In Japanese Patent Application Kokai (Laid-Open) Nos. 60-26024, 61-120816, 63-317520, 1-24826, 1-132626 (EP 317262), EP 303,246 and U.S. Pat. No. 4,614,778, it is disclosed that thermoplastic saturated norbornene type polymers such as hydrogenated product of ring-opened polymer of norbornene type monomer and addition type copolymer of norbornene type monomer and ethylene have excellent characteristic features as optical disk substrate material. However, even if these polymers are used as substrate, the aluminum constituting the reflecting layer is still similarly susceptible to oxidative deterioration. Although vapor deposition of gold resistant to oxidation is practically performed, it is not advantageous industrially because of its expensiveness.